1. Field of the Invention
The present invention relates to a gas recovery unit, in which a CVD device is provided which has a gas introduction part having an inert gas supply path for supplying a diluting inert gas and a cleaning gas supply path for supplying a cleaning gas, each connected thereto. The unit also has an exhaust gas discharge part for releasing an exhaust gas, and an exhaust gas recovery part for cooling down and recovering said cleaning gas from the exhaust gas from said CVD device and an exhaust gas purification part for purifying the liquefied and recovered exhaust gas. The present invention also relates to a gas recovery unit in which an etching device (A) is provided which has a gas introduction part having an inert gas supply path for supplying a diluting inert gas and an etching gas supply path for supplying an etching gas, each connected thereto. The unit also has an exhaust gas discharge part for releasing an exhaust gas, and an exhaust gas recovery part for cooling down and recovering said etching gas from the exhaust gas from said etching device and an exhaust gas purification part for purifying the liquefied and recovered exhaust gas.
2. Description of the Prior Art
In a gas recovery unit, in which a CVD device is provided which has a gas introduction part having an inert gas supply path for supplying a diluting inert gas and a cleaning gas supply path for supplying a cleaning gas, each connected thereto, and which has an exhaust gas discharge part (A7) for releasing an exhaust gas, it is necessary to remove, from said CVD device, foreign matters generated therein such as silicon dioxide (SiO.sub.2), polysilicon, silicon nitride, metal silicide and amorphous silicon, because such foreign matters can have a detrimental influence upon the products. Accordingly, there has been hitherto carried out such a cleaning method which comprises introducing a cleaning gas into said CVD device to convert said foreign matters to volatile substances, and removing these volatile substances. By introducing into said CVD device a compound which can be reacted with said foreign matters to convert said foreign matters to gaseous components, such as nitrogen trifluoride (NF.sub.3) which converts silicon dioxide (SiO.sub.2) to silicon fluoride (SiF.sub.4), same has to be removed from the CVD device. The cleaning gas has also been often used in an amount larger than its necessary amount, and in such a case, this excess amount of the cleaning gas beyond the necessary amount will be mixed in said exhaust gas. When said cleaning gas is in particular harmful or expensive, it is desired that the cleaning gas be recovered for recycle use. Thus, various gas recovery units have been proposed.
Also, in using an etching device, in which an etching gas capable of being reacted, for example, with a semiconductor film or the like is brought into contact with said semiconductor film, thereby carrying out the etching, recovery of the etching gas for recycle use has been suggested.
There has been proposed as the exhaust gas recovery part of such a gas recovery unit a recovery part in which the cooler is constructed by providing a supply part for supplying cooling gas in a liquid state and a cooling gas circulation part which will be cooled down by heat of evaporation of said cooling gas. As the exhaust gas purification part of such a gas recovery unit, it has been suggested to construct one so that the recovered exhaust gas is purified by use of a rectification column having a condenser accommodating the cooling gas in a liquid state. Even if a gas recovery unit is designed which has the aforementioned exhaust gas recovery part and exhaust gas purification part, however, there are various problems. For example, various problems include the power required, the unit as a whole easily becoming larger in scale, the increased cost for operating the cooler and condenser, and the cooling efficiency becoming lower in relation to energy consumption because the cooler and condenser are comprised in such a way that the evaporated cooling gas is used as recycle by way of a compressor for liquefying the same gas again. Thus, it has been further proposed to use liquid nitrogen as said cooling gas.
In such a conventional gas recovery unit as discussed above, however, the design has been such that the nitrogen gas formed by the evaporation of liquid nitrogen is released for disposal to the atmosphere, although the cooling efficiency can be set relatively higher. Therefore, it has been pointed out that it is uneconomical to release the evaporated nitrogen gas for disposal to the atmosphere. Further, it is desired to improve the gas recovery unit so that the cleaning gas or etching gas can be more effectively recovered from the exhaust gas.
Due to the consideration of the aforementioned facts, it is therefore an object of the invention to provide a gas recovery unit in which the recovery efficiency of the cleaning gas or etching gas is greater and the running cost is less expensive.